1. Field of the Invention
This invention relates to a strain detector for detecting strain in a driven shaft such as, for example, a rotary shaft when an external torque or force is applied to the shaft.
2. Description of the Prior Art
Various strain detectors for detecting strain in a driven shaft are conventionally provided. An exemplary one of such detectors is disclosed, for example, in Japanese Kopai No. 57-211030, as shown in FIG. 8. Referring to FIG. 8, a driven shaft 40 in the form of a rotary shaft which is an object for the detection of strain is made of a non-magnetic material and supported for rotation around a center axis 46 by means of a pair of bearings 41 and 42. The bearings 41 and 42 are made of a non-magnetic material and supported on support members 43 and 44, respectively, which are also made of a non-magnetic material. A magnetic layer 45 made of a soft magnetic material having a high permeability is fixedly mounted on an outer periphery of the driven shaft 40. The magnetic layer 45 is composed of a plurality of parallel layer stripes which extend at an angle of -45 degrees with respect to the center axis 46. A cylindrical coil bobbin 47 made of a non-magnetic insulating material is disposed around the driven shaft 40 and supported on the support members 43 and 44. A detecting coil 48 is wound on the bobbin 47 and includes a pair of coil sections 48a and 48b.
With the strain detector of the construction described above, if an external torque or force is applied to the driven shaft 40, then strain is produced on the magnetic layer 45 to change the permeability of the driven shaft 40. The detecting coil 48 detects a variation of the permeability of the magnetic layer 45 as a variation in magnetic impedance, and a detecting circuit 14 (FIG. 9) detects and develops a strain detection output.
If a disturbance magnetic field from the outside acts upon the strain detector described above, such disturbance magnetic field tends to penetrate into the inside of the detecting coil 48. Here, since the drive shaft 40, bearings 41 and 42 and support members 43 and 44 are all made of non-magnetic materials, magnetic fluxes of the penetrating disturbance magnetic field pass through the magnetic layer 45 made of a magnetic material. Consequently, the magnetic operating point of the magnetic layer is displaced, which will cause an error in the output of the strain detector. Therefore, it is helpful to differentially amplify outputs of the coil sections 48a and 48b of the detecting coil 48 to cancel an influence of the disturbance magnetic field. However, since the magnetic characteristic is not symmetrical an error arising from a disturbance magnetic field is not removed completely, and accordingly, a sufficiently high degree of accuracy cannot be attained and stabilized detection of strain cannot be attained.
Another conventional strain detector is shown in FIG. 9. Referring to FIG. 9, a driven shaft 1 in the form of a rotary shaft is supported for rotation around a center axis 2 by means of a pair of bearings 3 and 4. A pair of elongated magnetic layers 5 and 6 made of a soft magnetic material having a high permeability and suitable magnetostriction are fixedly mounted on an outer periphery of the driven shaft 1 in a spaced relationship from each other in an axial direction of the driven shaft 1. Each of the magnetic layers 5 and 6 is composed of a plurality of parallel layer stripes which extend at angles of +45 degrees and -45 degrees with respect to the center axis 2, respectively. A cylindrical coil bobbin 7 is supported on the bearings 3 and 4 and disposed around the magnetic layers 5 and 6 in a concentric relationship to the driven shaft 1. A pair of detecting coils 8 and 9 are wound on the coil bobbin 7 corresponding to the magnetic layers 5 and 6, respectively. The detecting coils 8 and 9 are connected to a detecting circuit 14. A pair of yokes 10 and 11 made of a PC Permalloy containing about 80 percent by weight of nickel are disposed around the detecting coils 8 and 9, respectively. A first non-magnetic shield 12 made of a non-magnetic material having a high electric conductivity such as copper or aluminum is disposed commonly on outer peripheries of the yokes 10 and 11. A second magnetic shield 13 made of a soft magnetic material having a high permeability such as PC Permalloy is disposed around the first shield 12.
With this construction, if an external torque or force is applied to the driven shaft 1, then a tensile force is produced on either one of the magnetic layers 5 and 6 while a compression force is produced on the other of the magnetic layers 5 and 6, thereby causing the magnetic layers 5 and 6 to be strained. The magnetostriction of the magnetic layers 5 and 6 allows the orientation of the magnetization within each domain to be altered by such strain, which varies the permeability of the magnetic layers 5 and 6. In this instance, the permeability is varied in the opposite direction whether the strain is caused by a tensile force or a compression force. Each of the detecting coils 8 and 9 detects a variation of the permeability of the detecting coil 8 or 9 as a variation in magnetic impedance, and the detecting circuit 14 receives outputs of the detecting coils 8 and 9 and develops a detection voltage V corresponding to an amount of strain of the driven shaft 1. The yokes 10 and 11 converge magnetic fluxes produced by the detecting coils 8 and 9 to prevent leakage of magnetic fluxes from the detector to improve its sensitivity. Since the first shield 12 is formed from a non-magnetic material having a high electric conductivity, the depth of penetration of alternating magnetic fluxes is reduced to a very small value. Consequently, internal magnetic fluxes and external magnetic fluxes are magnetically separated from each other by the first shield 12. Accordingly, leakage of internal magnetic fluxes is prevented to raise the sensitivity of the detector while invasion of an external magnetic field into the detector is prevented to improve its noise preventing property. Further, since the second shield 13 is formed from a PC permalloy, it prevents invasion mainly of external direct current magnetic fluxes into the detector.
With this strain detector, however, since the first shield 12 has a simple cylindrical configuration, it cannot sufficiently prevent axial invasion of an external alternating current magnetic field and axial leakage of an internal magnetic field into and from the detector. Consequently, external magnetic fluxes, particularly direct current magnetic fluxes, flow though the magnetic layers 5 and 6 so that the operating point of each of them is displaced to cause an error in the detection of strain by the detector, and to deteriorate the sensitivity of the distortion detector. Further, where the detecting coils 8 and 9 are disposed close to each other, a magnetic interference takes place between them, which causes an error in detection of distortion. Accordingly, miniaturization of the distortion detector is difficult.